The present invention relates to a digital receiver for multi-frequency signals with pulse code modulation and more particularly to a device for recognizing each of the frequencies to be detected.
Such multifrequency signals are particularly used for signalling between automatic telephone exchanges or between subscribers and telephone exchanges. The signalling codes, called code R.sub.2 or code MF SOCOTEL in the first case and "keyboard" code in the second, are most often formed from two frequencies belonging to a group of N frequencies, one frequency only being present for the monitoring frequency of the MF SOCOTEL code. The purpose of the multi-frequency digital receivers is to detect the presence of these frequencies in a signal, these frequencies sometimes being called reference frequencies.
Applicants' French Pat. No. 2 299 769 already discloses a digital device receiving multi-frequency signals. It consists essentially in effecting the intercorrelation of the incoming signal with signals indicative of reference frequencies, said signals being in sine and cosine form and being stored in the form of samples in read-only memories, compressed according to a logarithmic law. A threshold detector compares at the output the magnitude of the result of calculation determining the detection of a frequency.
The present invention also employs the functions of intercorrelation of the incoming signal E with signals indicative of the reference frequencies placed in sine and cosine form but previously weighted by time windows. The use of time windows has already been described in earlier documents, e.g. the article IEEE Trans. on Communications of December 1973 by Messrs. KOVAL and GARA describes application thereof to the time filtering of signals by means of time windows which weight the samples of the incoming digital signal E.sub.n, as a function of their rank, in order to attenuate the effect of the adjacent frequencies.
A time truncation brings about a distorsion in the harmonic analysis of the signal. The article by F. J. HARRIS in Proceedings of the IEEE Vol. 6, no. 1, January 1978 describes the effect of the windows on the spectral distribution by Fourier transform of a signal.
In multifrequency digital receivers, the function of the truncations is to eliminate the influence of the frequencies detrimental to the algorithm of decision. To this end, the selected windows must present weak lateral lobes in order to reduce the affect of signals belonging to other frequency bands of the spectrum.
It was also usual to choose time windows with a narrow central lobe; in particular, if f.sub.i and f.sub.j are two reference frequencies for the selected code, and adjacent, it was appropriate to adopt a window of width such that the distance e between the centre of the central lobe and one of its edges, of infinite attenuation, is less than .DELTA.f.sub.ij, the distance between the two nominal frequencies f.sub.i, f.sub.j. Such a characteristic has been described for example in French Pat. No. 2 424 669, page 7, lines 15-25. FIG. 1 shows this criterion of the prior art of discernability of two frequencies f.sub.i and f.sub.j (e&lt;.DELTA.f.sub.ij).
The invention proposes to select time windows wherein around a frequency f.sub.i at the immediately adjacent frequencies f.sub.i.+-.1 there is not an infinite attenuation of the central lobe, but, on the contrary, there is an attenuation greater than or equal to a predetermined value K at f.sub.i.+-.1, but infinite attenuation at the following frequencies f.sub.i.+-.2.
Such a choice gives the time windows of the invention a new function of recognition of a frequency f.sub.i by establishing values V (f) for adjacent frequencies f.sub.i+1 and f.sub.i-1, the values V(f.sub.i), V(f.sub.i+1) and V(f.sub.i-1) conserving predetermined relations with one another.